1. Field of the Invention
The present invention relates to a dispersion element, a spectral device, and a wavelength selective switch.
2. Description of the Related Art
A method for manufacturing an immersion grating (dispersion element) is disclosed in “Low polarization dependent diffraction grating for wavelength demultiplexing, E. Popov et al., Optics Express, Vol. 12, Issue 2, pp. 269-275.” In this method, a grating pattern is formed on a silicon wafer by using a phase shift mask, and then grooves are formed by means of crystallographic etching (tetramethylammonium hydroxide crystallographic etching) using tetramethylammonium. These grooves are finely etched on one side of the wafer that is an extremely high-precision plane parallel plate. Hereinafter, a silicon wafer on which grooves are formed is called a chip.
As a conventional wavelength selective switch, a reflective wavelength selective switch is disclosed in the specifications of U.S. Pat. No. 6,707,959.
FIGS. 16 and 17 are schematic views illustrating how light passes inside a conventional immersion grating (dispersion element). In the conventional immersion grating shown in FIGS. 16 and 17, a side surface 921 of a separately provided prism 920 is cemented to a surface 902 without grooves of a chip 900. On the chip 900, the grooves are formed on a surface 901 arranged opposite to the cemented surface 902. In this immersion grating, light is input from another side surface 922 of the prism 920.
However, conventional cementing medium cannot be used to cement the chip and the prism. This is because an appropriate cementing medium does not exist due to the fact that the refractive index of silicon is higher than 3 in the wavelength of near infrared rays.
Therefore, for cementing the chip and the prism, it is desirable to use a normal temperature cementing such as optical contact and activated cementing.
The most part of a light L80 or a light L90 that enters the immersion grating shown in FIG. 16 or 17 are dispersed at the surface 901, which is a diffraction plane, after that, enters the prism 920 without reflection at the surface 902 and the side surface 921, which function as a cementing surface where the chip 900 and the prism 920 that are cemented together, and is launched from the side surface 922 that functions as both an incident surface and an output surface. The light L81 (solid line in FIG. 16) or the light L91 (solid line in FIG. 17) that is launched from the side surface 922 is called an ordinary light.
When the cementing between the chip 900 and the prism 920 is not perfect and foreign substances such as impurities enter the cementing surface, a part of light that passes through the cementing surface and is dispersed at the surface 901 is reflected at the cementing surface even when the medium on both the sides of the cementing surface is the same. Out of the light reflected on the cementing surface, the light L81 (solid line in FIG. 16) or the light L91 (solid line in FIG. 17) that has diffracted only once at the diffraction plane 901 is launched in the same direction as that of the ordinary light L81 or L91 from the side surface 922 of the prism 920. The light L81 or L91 that is launched from the side surface 922 after being reflected at the cementing surface is called a noise light.
When such an immersion grating is used in a spectral device having a detector array, the ordinary light and the noise light travel in parallel after being launched from the immersion grating, converge by a lens or a reflected light system, and both of them enter the same detector.
When the immersion grating is used for a wavelength selective switch, both the ordinary light and the noise light enter a switching element of the same channel.
When a coherent light is used as an incident light for the immersion grating, because the ordinary light (signal light) and the noise light have an optical path difference, the ordinary light and the noise light cause interference leading to an intensity modulation. Moreover, because the optical path difference differs depending on the wavelength, the intensity changes depending on the wavelength. This phenomenon is undesirable for a spectral device or a wavelength selective switch.
Moreover, this phenomenon occurs not only at a cementing surface of silicon medium, but also at a cementing surface of high refractive index optical mediums as well.